Another writing implement I picked up on a whim, the Pilot Presso Side Knock mechanical pencil has a lot of good going for it. Aside from the lead advancement button placed handily on the pencil barrel, the eraser is what really sold me on this model. JetPens explains the design better than I can:
The side button is located higher up on the pencil body so it doesn't interfere with your grip of the pencil. Presto! You can extend lead with a convenient click of a side button. Plus, the pencil comes in six beautiful see-through colors including green, blue, orange, pink, soft blue and yellow.
The sizable eraser really comes in handy. At just $3.00, you are getting your money's worth just from the non-business end of the pencil.